1. Field of the Invention
The invention generally relates to methods and an apparatus for taking underwater spectroscopic measurements. In particular, the invention relates to the measurement and identification of contaminants in sediment layers by analysis of luminescent signatures of contaminants.
2. Brief Description of Related Art
Some existing sensor systems can acquire in-situ luminescence (fluorescence and phosphorescence) or UV/visible data in the field from soils or sediments. Of particular interest are sensors focused on penetration of solid-liquid media followed by in-situ imaging and measurement.
Fiber optic in-situ sensors have been developed for measuring fuels and other organic chemicals in terrestrial soils. No sensors have been developed for deployment in aquatic media for the purpose of sensing within the seabed, lake bed, or the bottom of rivers. The in-situ soil sensors generically consist of (1) an above ground power source, incandescent or laser excitation light source, a light control unit, data logger, display and processing sensor unit; and (2) a fiber optic cable which serves to both transmit the excitation light into the soil and collect and transmit the signal up to the sensor unit, the fiber optic cable being inserted into the soil by means of a cone penetrometer or is lowered into a pre-drilled test hole.
A recent review of fiber optic chemical sensors is presented in Klainer et al., Environmental Monitoring Applications of Fiber Optic Chemical Sensors (FOCS), Vol. II, Chapter 12, CRC Publishers, (1991). Two portable prototype spectrometers are described. One of these consists of an incandescent lamp illumination source and a photodiode detector. The emitted light entering the optical splitter is filtered so only one wave length band is emitted. This excitation wave length band is then split by a dichroic mirror separating the excitation from the UV emission bands. The signal channel is detected by a photodiode. The area of measurement is dictated by the diameter of the optical fibers and can be from 100 to 600 .mu.m in diameter (Milankovich, Daley, Klainer and Eccles, "Remote Detection of Organochlorides With Fiber Optic Based Sensor, II. A Dedicated Portable Fluorimeter", Anal. Instrum., 15, 347 pp., 1986).
The second portable spectrometer described by Klainer et at., (1991) is designed to measure fluorescence or change in refractive index (Douglas Instruments Co., Palo Alto, Calif.). This spectrometer uses a miniature optical bench, a small adjustable output tungsten halogen lamp, an adjustable light path for critical alignment, optical band-pass filters to provide a narrow excitation beam, a dichroic beam splitter, and a photodiode to respond to 300 to 1600 nm excitation wave lengths. The excitation signal is passed through a preamplifier to a signal data acquisition system.
Another view of portable spectrometers is given in Eastwood, Lidberg, Simon and Vo-Dinh "An Overview of Advanced Spectroscopic Field Screening and In-Situ Monitoring Instrumentation and Methods," Chemistry for the Protection of the Environment, Plenum Press, New York, (1991) and Eastwood and Vo-Dinh, Molecular Optical Spectroscopic Techniques for Hazardous Waste Site Screening, EPA/600/S4-91/011, September, 1991. The authors indicate that, at the date of publication, only two truly portable fluorimeters existed. One is the Baird Field Identification Luminoscope Monitor (FILM), manufactured for the Coast Guard to detect oil and hazardous chemicals in water. It operates off a 12 v dc battery and uses a mercury lamp with a 254 nm line isolated for excitation. The emission spectrum (250 to 600 nm) is dispersed by a flat field grating and recorded on polaroid film. The second portable system is called the L-101A Fiber Optic Luminoscope (Environmental Systems Corporation). This system also uses a mercury lamp for excitation but isolates the 365 nm or other Hg line. The excitation light is focused onto a bifurcated fiber-optic lightpipe and is transmitted to a probe which can be placed against soil surfaces or into water. The returning emission is then directed into a manually scanned monochrometer with photomultiplier detection.
Another fiber optic system, not described in the above reviews, is the pulsed nitrogen laser fiber-optic fluorimeter (Inman, Thibado, Theriault and Lieberman, "Development of a pulsed-laser, fiber-optic-based fluorimeter: determination of fluorescence decay times of polycyclic aromatic hydrocarbons in seawater", Analytica Chimica Acta, Elsevier Science Publishers B. V., Amsterdam, 239 (1990) pp. 45-51. This system uses excitation light provided by a pulsed nitrogen laser with a pulse width of 3 ns and a pulse energy of 300 .mu.J. The pulsed light is passed through a beam splitter and about 80% of this light enters a 10 m length of 325 .mu.m diameter UV-grade fiber. Some of the excitation light is passed through a beam splitter and directed toward an avalanche photodiode to determine the time of firing of the laser in order to gate the detector. Six receiver fibers are concentrically arrayed around the central excitation fiber. The emission signal is then transmitted through collimating optics and the signal is fed into a compact spectrograph with a 300 groove/mm holographic grating. A filter blocks the emission spectrum below about 400 nm. The emission spectra are measured with a diode array. The output yields a spectral range of ca. 350 nm with a resolution of 0.5 nm/pixel. The time-resolved analysis requires binning of pixels by 10, decreasing the spectral resolution to 5 nm but reducing noise and increasing sensitivity. This system has been coupled to a cone penetrometer and is pushed into compact soils by a hydraulic piston (Apitz, Theriault, and Lieberman, "Optimization of the optical characteristics of fiber-optic guided laser fluorescence technique for the in-situ evaluation of fuels in soils", SPIE, The International Society for Optical Engineering, Proceeding Vol., 1637, present at OE/LASE, 22 Jan. 1992, Los Angeles, Calif.). This general concept of integrating a spectrometer into a soil penetrating probe is covered in U.S. Pat. No. 5,128,882 granted to Cooper and Malone in 1992. In both the Apitz and Theirault, and Lieberman, 1992 system and that described in the Cooper and Malone, the spectrometers measure luminescence through an optical fiber over a small diameter window. Neither of these systems contain reference standards within the window probe.
A chemical electroluminscence electrode (U.S. Pat. No. 5,075,172 to Dixon et al., granted 1991) has been developed for rapid screening of contaminated ground water for aromatic hydrocarbons (esp. benzene). This system consists of an electrochemical optical fiber; an optrode sensor coated with ruthenium bipyridyl which registers an increase in light emission (500 to 700 nm) when in contact with aromatic hydrocarbons in ppb (parts per billion) concentrations (Dixon et al., "Electrochemiluminescent Optrode Development for the Rapid and Continuous Monitoring of Petroleum Contamination", Petroleum Contaminated Soils, Vol. 3, (Kostecki, et al., editors) chapter 10, pp. 111-124, Lewis Publishers, Chelsea, Michigan 48118, 1990). A proposed (not existing) prototype field instrument would consist of a small PVC pipe containing the optrode sensor and light detector electronics that could be lowered into a test well. All recording equipment is located above ground.
Non-fiber optic based field sensors have been developed to measure volatile organic compounds (VOC) in soils at Superfund Sites. These include Purge-and-Trap Gas Chromatography. The Purge-and-Trap or continuous flow gas system may also be coupled with fiber optic sensors (Leonard and Tillman, "SENSOR INTEGRATION FOR SITE SCREENING: Smart Weapons for the Fight Against High Costs", present at Third International Symposium: Field Screening Methods for Hazardous Wastes and Toxic Chemicals, held February 24-26, 1993, Sands Hotel, Las Vegas, Nev.). This involves the insertion of a gas permeable sampling vessel into the ground via a penetrating tool. Several of these can be deployed over the area of interest. A soil-gas sampling line connects the gas permeable sampling vessel to the surface where a sampling manifold allows for on-line intake of gas samples from different collection points. An injector inserts the sample into a portable gas chromatographic analyzer and analysis is done in real time. (Volatile Organic Compounds in Water by Purge and Trap Capillary Column Gas Chromatography with Photoionization and Electrolytic Conductivity Detectors in Series, Method 502.2, U.S. EPA, Cincinnati, 1986).
Many aquatic environments contain particulate organic matter which eventually settles forming a deposit on the bottom. Some of these particles can be associated with organic contaminants including hydrocarbons. Assessment of organic pollution of the seafloor, lake, or river bottom is required in many environmental monitoring studies. Once particle associated contaminants settle to the bottom, they may be buried by new sediment or biologically mixed Coioturbated) into the bottom to depths of a few 10's of centimeters. Sediment quality assessment therefore requires imaging and measurement of contaminates within the biologically active zone of the bottom (usually .ltoreq.25 cm). Sources of contamination may be inferred from the spatial relationship of fluorescent contaminants to imaged dredge material layers, sewage sludge layers, oil globules, or deposits proximal to industrial/municipal effluent, or atmospheric sedimentation from combustion sources.
This disclosure addresses the need for developing a rapid chemical screening tool by describing an instrument that can collect, in near real-time, in situ chemical information from an upper sediment column. This field reconnaissance measurement technique may be used to map temporal/spatial gradients of organic contaminants in aquatic sediments.
This invention is based on the concept of sediment-profile imaging as first described and practiced by Rhoads and Cande "Sediment Profile Camera For In Situ Study of Organism-Sediment Relations," Limnology and Oceanography, Vol 16, No. 1, pp. 110-114, (1971). This invention improves and extends the concept to include UV spectrometry (fluoroscopy). A detailed description of the Rhoads-Cande camera was publicly disclosed in the 1971 journal reference and a patent was never applied for. The Rhoads-Cande profile camera was based on a still film camera using off-the-shelf black and white negative or color transparency film and visible light for illumination. The Rhoads-Cande photographic profile camera is used to image biological and physical structures in the upper 20 cm of the bottom but the system does not have the capability to make chemical measurements.